freesewing/markdown/org/docs/about/faq/notches/en.md

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Why don't you use double notches to mark the back of the pattern?

A double-notch is a common way to mark the back of the pattern. Common, but not universal.

Different parts of the world have different conventions or traditions on how to notch patterns. To avoid confusion, we use a differently styled notch to indicate the back, as detailed in our pattern notation guide.